Non-radioactive IDU (5-iodo-2'-deoxyuridine), while not teratogenic to rats, does produce foetal malformations in rabbits when administered topically to the eye in doses similar to those used clinically, 0.1 percent four times a day for twelve days. These malformations include exophthalmus and clubbing of the forelimbs. By contrast, F3TdR (trifluorothymidine), another highly effective anti-herpetic agent currently under investigation but not available for general use, was found not to be teratogenic to rabbits, even when given in concentrations ten-fold greater than the doses used to produce IDU teratogenicity. Under the conditions of this study, systemically or topically applied Thimerosal was found to have no teratogenic effect in rabbits and rats.